1. Field of the Invention
The present invention relates generally to disk format controllers for the frequency modulation/modified frequency modulation (FM/MFM) recording format as employed in MS-DOS operating systems and the group code recording (GCR) format as employed in systems developed by Apple Computer, Inc. More specifically, the present invention is concerned with a copy protection technique for preventing unauthorized copying of original software products.
2. Description of the Related Art
Since recordings on disks, either magnetic or optical, are made as a bit serial stream on a single track, special provision is made to allow the reading system to maintain synchronization. This is achieved by the encoding format, which either includes clock pulses or encodes the data in such a way that there cannot be a case where there is an excess number of no transition states, or 0s. In FM/MFM recording, a clock pulse is used to represent a transition and in a GCR format a data stream is encoded by breaking it into groups of four bits and mapping these onto five bit groups. Due to the resultant redundancy, the five bit groups are selected to limit the number of consecutive 0s to control the maximum spacing between transitions.
Since the recorded clock pulses are used exclusively to maintain synchronization, the current disk format controller of the MS-DOS system exclusively outputs data pulses to the host system, where the original application program can be illegally copied. In the GCR format system, data pulses read from a disk are decoded by the disk format controller according to the GCR decoding rule before transferred to the host system, where illegal copying may occur.
To prevent the unauthorized duplication of original software products, attempts were made by software developers. These include changing the cyclic redundant check (CRC) sequence of a program recorded in FM/MFM format so that the MS-DOS system cannot easily duplicate the program, or changing the check sum bytes of a program in GCR format into a special format so that the program cannot easily be duplicated by the Macintosh Operating System developed by Apple Computer, Inc.